Overview
Explains managing private autonomous system numbers in BGP to ensure proper route handling and policy enforcement.
BGP private AS number management is a feature that
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allows routers belonging to a private Autonomous System (AS) to access the global Internet
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removes private Autonomous System Numbers (ASNs) from the AS path in outgoing update messages, and
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optionally replaces those numbers with the local router's ASN to maintain AS path length.
Private AS numbers are ASNs that
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are used by Internet Service Providers (ISPs) and customer networks to conserve globally unique ASNs
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cannot access the global Internet because they are not unique, and
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range from 64,512 to 65,535.
Private AS number removal and replacement in BGP
Public ASNs are assigned by InterNIC, range from 1 to 64,511, and are globally unique. Private ASNs range from 64,512 to 65,535, are not unique, and cannot appear in the global BGP routing table.
Because BGP best path calculations require unique ASNs, private ASNs must be removed from the AS path before propagating routes to external peers. With this featurem you can configure routers to strip private ASNs from outgoing updates in external BGP (eBGP) sessions. Optionally, the router can replace them with its own ASN to preserve the AS path length.